This invention concerns derivatives of an indolocarbozole alkaloid referred to as "K-252a," and the use of these derivatives to enhance molecular, biological and cellular activities which result from the binding of neurotrophins to cells which comprise neurotrophin receptors.
The cause of neurodegenerative disorders such as Alzheimer's, Parkinson's and Amyotrophic Lateral Sclerosis (ALS or "Lou Gehrig's Disease") is unknown. In recent years, however, it has been hypothesized that alterations in neurotrophin localization, expression levels of neurotrophins, and/or expression levels of the receptors which bind the neurotrophins, may accompany such disorders. Neurotrophins are low molecular weight polypeptides that play a role in the development, function, and/or survival of certain cells, including neurons. The death or dysfunction of neurons have been directly implicated in a number of neurodegenerative disorders.
A variety of neurotrophins have been identified. These include Nerve Growth Factor (NGF), Neurotrophin-3 (NT-3), Neurotrophin-4/5 (NT-4/5), and Brain Derived Neurotrophic Factor (BDNF). Of these, NGF was the first neurotrophin identified and is the best characterized neurotrophin.
NGF is required for the normal development and function of certain sensory and sympathetic neurons. Experimental evidence to date indicates that NGF regulates a variety of cellular responses important to the function of neurons. The present lack of definitive information regarding the in vivo function of other neurotrophins, such as NT-3, has hampered those investigating the causes of neuro degenerative disorders. It is presently known that NT-3 plays a role in the survival and function of cholinergic neurons in basal forebrain. However, as with NGF, NT-3 may also influence the survival and/or function of many different cell types.
It has been suggested that the lack of or inadequate functioning of neurotrophins is linked to neuronal degeneration. This degeneration occurs in disorders such as Alzheimer's, Parkinson's and ALS. Accordingly, it has been suggested that by providing those suffering from such disorders with a corresponding neurotrophic factor, such neural degeneration may be alleviated or prevented. (See, Appel, S. H. 10 Ann.Neurol. 499, 1981). For example, it has been suggested that NGF may be useful in the treatment of Alzheimer's because NGF is the trophic factor for the population of basal forebrain cholinergic neurons which degenerate in Alzheimer's patients. As suggested, "adding" NGF to a patient's system may prevent the death of such neurons, or improve the function of such neurons.
Unfortunately, technical and ethical considerations have so far hampered the development of therapeutic agents based upon the neurotrophins. For example, it is technically difficult to produce sufficient quantities of pure neurotrophins using so-called recombinant DNA techniques. Additionally, although it is possible to utilize human fetal cells to produce neurotrophins, the ethical ramifications raised by the use of such cells (typically obtained from an aborted fetus) has all but prevented the utilization of this approach. Accordingly, the search for a different approach or approaches to the use of neurotrophins in the mediation of disorders or diseases has intensified.
A more refined understanding of the role of the neurotrophins in neurodegenerative disorders would be useful in addressing such approaches. Such an understanding would allow for a more rational design of therapeutics because once the mechanistic relationship between the neurotrophins and a specified disorder is fully appreciated, then defects, flaws or problems in such mechanisms can be more appropriately addressed.
One such approach would involve the development of molecules which "mimic" the effect of the neurotrophins; for example, molecules which bind to neurotrophin receptors, whereby such binding leads to certain cellular responses identical to those achieved when the neurotrophin-like molecule binds to its receptor. Another approach would involve the development of molecules which enhance the response or responses associated with the binding of the neurotrophins to their receptors, i.e., molecules which, in effect, mediate neurotrophin-receptor interaction that is otherwise not viable or is insignificant.
Focusing on the latter approach, it is known that the binding of neurotrophins to their receptors (these receptors are collectively referred to as "trk ") leads to the immediate phosphorylation of tyrosine residues on the receptor, referred to as "autophosphorylation". Autophosphorylation is an absolute requirement for the activation of signal transduction pathways which regulate certain molecular responses which can relate to the function and/or survival of cells such as neurons. (See, generally, Klein et al. 65 Cell 189, 1991; and Lamballe et al. 66 Cell 967, 1991). The trk family of receptors currently comprises trk A, trk B and trk C. Individual trk receptors bind with different subsets of neurotrophins. For example, NGF binds trk A, but not trk B; NT-3 binds primarily trk C, and, to a lesser extent, trk A (however, NT-3 binding to trk A results in little, if any, observable functional activity). The importance of trk A in the trk family and the molecular mechanisms which result from trk A tyrosine kinase activity are manifest because of the putative connection between NGF and certain neurodegenerative disorders.
Maintaining this focus, it would be highly desirable to identify agents which enhance neurotrophin induced activity. Such agents would provide a benefit in further understanding and exploiting the molecular mechanisms of the neurotrophins, as well as for use as therapeutic agents for the treatment of neurodegenerative disorders.